Analysis of the Campylobacter jejuni FlgR Response Regulator Suggests Integration of Diverse Mechanisms to Activate an NtrC-like Protein

Department of Microbiology, University of Texas Southwestern Medical Center, Dallas, TX 75390

^* To whom correspondence should be addressed. Email: david.hendrixson{at}utsouthwestern.edu.

	Abstract

Flagellar motility in Campylobacter jejuni mediates optimal interactions with human or animal hosts. ⁵⁴ and the FlgSR two-component system are necessary for expression of many C. jejuni flagellar genes. The FlgR response regulator is homologous to the NtrC family of transcriptional activators. These regulators usually contain an N-terminal receiver domain, a central domain that interacts with ⁵⁴ and hydrolyzes ATP, and a DNA-binding C-terminal domain. Most often, phosphorylation of the receiver domain influences its inherent ability to either positively or negatively control activity of the regulator. In this study, we performed genetic and biochemical analyses to understand how FlgR activity is controlled to culminate in expression of ⁵⁴-dependent flagellar genes. Our data suggest that the FlgR receiver domain has the capacity for both positive and negative regulation in controlling activation of the protein. Analysis of the C-terminal domain of FlgR revealed that it lacks a DNA-binding motif and is not required for ⁵⁴-dependent flagellar gene expression. Further analysis of FlgR lacking the C-terminal domain indicates that this protein is partially functional in the absence of the cognate sensor kinase, FlgS, but its activity is still dependent on the phosphorylated residue in the receiver domain, D51. We hypothesize that the C-terminal domain may not function to bind DNA but may ensure specificity of phosphorylation of FlgR by FlgS. Our results demonstrate that FlgR activation mechanisms are unusual amongst characterized NtrC-like proteins and emphasize that varied means are utilized by the NtrC family of proteins to control transcription of target genes.